1. Field of the Invention
The present invention relates to a pneumatic tire including a tread with a rib pattern.
2. Background Art
There have been proposed various structures for a rib pattern included in a tread of an pneumatic tire. Examples thereof include a pattern structure disclosed in JP 2009-292252 A.
As shown in FIG. 1 of JP 2009-292252 A, this pattern structure includes a rib formed, in the center region of the tread, as a land area interposed between two main grooves extending in the tire-circumferential direction. A narrow groove extending in the tire-circumferential direction is formed at the center in the tire-width direction of the rib (in coincidence with the tire equator). A plurality of curved grooves extend from each main groove toward the tire equator side. Each curved groove has one end opening into the corresponding main groove and the other end closed within the rib. In addition to this, a plurality of sipes (a plurality of grooves each having an extremely narrow width) extending from each main groove toward the tire equator are formed on the rib in the center region of the tread. Each sipe is formed continuously from one end to the other end, with one end connected to the corresponding main groove and the other end connected to the narrow groove. The sipe intersects the curved groove so as to divide it.
According to paragraph 0024 of JP 2009-292252 A, such intersection of the sipe with the curved groove can facilitate deformation of the curved groove with the rotation of the tire. This deformation of the curved groove can facilitate removal of snow stuck into the curved grooves. Therefore, this tire exhibits good performance on snow.
However, it is disadvantageous to facilitate the deformation of the curved groove, when driving a vehicle on a dry road. According to the invention of JP 2009-292252 A, the sipe is configured to intersect the curved groove so as to divide it for the purpose of enhancing the performance on snow. In such a configuration, the sipe and the curved groove intersect each other, thereby forming four corners surrounding the crossing point. In the case where the curved groove and the sipe intersect each other in driving a vehicle on a dry road, a corner on one side and a corner on the other side, with the sipe interposed therebetween as a boundary, of the rib having the curved groove and the sipe in the tire-circumferential direction (in the direction of rotation) deform with the rotation of the tire differently from each other, corresponding to the difference in how they make contact with the road. This causes the corner on the front side in the direction of the rotation (that is, the corner on the side that first comes into contact with the road) to be worn more significantly (intensively) than the corner on the back side in the direction of the rotation (that is, the corner on the side that afterwards comes contact with the road), with the one sipe interposed therebetween. Such wear causes the corner that first comes into contact with the road to be rounded. This is what is called uneven wear (particularly, this uneven wear is called “heel and toe wear”). Such uneven wear becomes significant as the travel distance of the vehicle increases. Increased uneven wear makes a level difference in the tread because of the rounded corner on the one side with respect to one sipe. This increases the driving noise. For this reason, there is a concern that the driver of the vehicle is made to feel uncomfortable.